Provide preference mapping for Java Execution Environments -> Runtimes

[fbricon]

We need a set of preferences so that you can define multiple Java runtimes, that can be different from the runtime used to launch jdt.ls.

eg.

"java.configuration.runtimes": {
"JavaSE-1.8": {
     path:"/path/to/jdk/1.8.301",
     sources:"/path/to/jdk/1.8.301/sources.jar",  //optional
     javadoc:"/path/to/jdk/1.8.301/javadoc.jar",  //optional
  },
"JavaSE-11": {
     path:"/path/to/jdk/11.0.301",
  }
}  

The keys would map 1:1 to the JDT Execution Environments.

[snjeza]

@fbricon Could we define the java.configuration.runtime property as

"java.configuration.runtimes": [
        {
            "name": "JavaSE-1.8",
            "path": "/path/to/jdk/1.8.301",
            "sources": "/path/to/jdk/1.8.301/sources.jar",
            "javadoc": "/path/to/jdk/1.8.301/javadoc.jar"
        },
        {
            "name": "JavaSE-11",
            "path": "/path/to/jdk/11.0.301",
        }
    ]

[fbricon]

ok, if it makes things easier

[fbricon]

Final syntax looks like :

"java.configuration.runtimes": [
        {
            "name": "JavaSE-1.8",
            "path": "/usr/local/jdk1.8.0_201"
        },
        {
            "name": "JavaSE-11",
            "path": "/usr/local/jdk-11.0.3",
            "sources" : "/usr/local/jdk-11.0.3/lib/src.zip",
            "javadoc" : "https://docs.oracle.com/en/java/javase/11/docs/api",
            "default":  true
        },
        {
            "name": "JavaSE-12",
            "path": "/usr/local/jdk-12.0.2"
        },
        {
            "name": "JavaSE-13",
            "path": "/usr/local/jdk-13"
        }
    ]

Runtime name must be one of:
"J2SE-1.5",
"JavaSE-1.6",
"JavaSE-1.7",
"JavaSE-1.8",
"JavaSE-9",
"JavaSE-10",
"JavaSE-11",
"JavaSE-12",
"JavaSE-13"

List will be updated with each supported release of the JDK